The present invention relates to fabrication cost and performance improvements in waveguide-fed slot array antennas.
Waveguide-fed slot array antennas are well known in the art. One type of this slot array antenna uses shunt coupled broad wall radiating slots.
An array of slot radiators disposed in a straight line along a wall of a waveguide is employed frequently to generate a beam of electromagnetic power. As a typical example of an array antenna composed of slot radiators, the antenna comprises a waveguide of rectangular cross section wherein the width of a broad wall is approximately double the height of a narrow wall, and wherein the slots are formed within one of the broad walls. Antennas are constructed also of a plurality of these slotted waveguides arranged side-by-side to provide a two-dimensional array of slot radiators arranged in rows and columns. To facilitate description of the antenna, a column of slot radiators is considered to be oriented in the longitudinal direction to a waveguide, in the direction of propagation of electromagnetic power, and a row of slot radiators is considered to be transverse to the waveguide. An antenna composed of a single waveguide generates a fan beam while an antenna composed of a plurality of the waveguides arranged side by side produces a beam having well-defined directivity on two dimensions.
Antennas employing slot radiators may have slots which are angled relative to a center line of the broad wall of the waveguide, or may have slots which are arranged parallel to the center line of the broad wall of the waveguide. In order to attain a desired linear polarization, and a desired illumination function of the radiating aperture of the entire antenna, the configuration of the antenna of primary interest herein is to be configured with all of the slots being parallel to each other.
A co-phasal relationship among the radiations from the various slot radiators is employed for generating a broadside beam directed perpendicularly to a plane containing the plurality of slot radiators. Herein, the antenna comprising the two-dimensional array of rows and columns of radiators with slots oriented in the column direction is of primary interest. One method of obtaining the co-phasal relationship is to position the slot radiators in alternating offsets fashioned along a centerline of each waveguide broad wall. The transverse offsetting of the slot radiator permits a coupling with a non-zero value of longitudinal component of the magnetic field of the electromagnetic wave in each of the waveguides. With a spacing of one-half guide wavelength along the direction of propagation within the waveguide, the alternation of the offsetting compensates for periodic variations in the phase of the magnetic field so as to obtain a constant value of phase in the radiated field. The waveguides are fed in phase and operate in the TE.sub.10 mode. Since the spacing and pattern of alternation of offsetting of slot radiators is the same in each of the waveguides, good control of the radiated beam is obtained without excessive grating lobes, i.e., energy radiating in unintended directions.
However, in the event that a TE.sub.n,0 mode rectangular waveguide, having a single broad wall with n columns and many rows of slots is employed in the lieu of the plurality of parallel slotted waveguides, then the relationship among the wave components in each of the columns changes. The phasing of the components of the wave in one column is 180 degrees out of phase with the wave components of the contiguous column. To compensate for this phasing of the wave components within the waveguide, the pattern of offset slot radiators of one column must be reversed from that of the contiguous columns of slots to ensure identity of slot phasing.
A problem arises in that the foregoing arrangement of reversed patterns of offset slot radiators introduces excessive grating lobes in addition to the desired beam. The resulting loss of antenna gain militates against the convenience of using a very wide waveguide with a single broad wall as an antenna, unless the grating lobes can be eliminated. This invention relates to a method to eliminate slot offsets while also reducing cost of fabrication.
The issue of eliminating slot offsets while maintaining producibility is the subject of U.S. Pat. Nos. 5,010,351 and 4,985,708 by the inventor of the present invention. The invention of U.S. Pat. No. 5,010,351 requires that an extra element in the form of an iris or vane be placed in the waveguide for each radiating slot employed. The invention of U.S. Pat. No. 4,985,708 requires that a thick and heavy plate be used for the wall of the waveguide to be slotted, and that slots be cut at an angle through that thick plate.